Such a stabiliser is known from Netherlands patent application No. 7307296. The desirable characteristics of a voltage stabiliser for use in a telephone set are: as low a direct current resistance as possible to keep the dissipation, particularly in the case of large line currents, at a minimum, a constant alternating current impedance which is high compared with the terminating impedance to avoid attenuation of the alternating current signals, such as speech and ringing signals, the alternating current impedance should in particular be independent of the consumed direct current, a low operating voltage to continue operation in the case of very long subscribers' lines is desired and a highest possible maximum line current to allow its use also in the case of very short subscribers' lines, without the maximum dissipation of the voltage stabiliser, which is preferably realized in integrated form, being exceeded.
In the stabiliser described in the above Netherlands Patent Application the alternating current impedance depends, however, to a very great extent on the direct current flowing through the stabiliser, owing to the so-called Early effect.
In addition, the stabiliser causes distortion of the alternating current signals, owing to the non-linear characteristics of the transistor circuit.